6 Cell and Tissue Architecture as a Checkpoint for EMT
Numerous studies point to post-translational control of EMT-TF
activity as critical in restricting EMT. GSK3β can phosphorylate
Snail to promote bTrcp-mediated ubiquitination [96]. Conversely,
lysyl oxidase-like 2 (LOXL2) can stabilize the Snail protein via
oxidative deamination of K98/K137 residues [97]. A recent
study identified a new regulation of Snail activity via the apicalbasal polarity machinery of mammary epithelial cells in 3D culture,
which acts to prevent EMT and consolidate the epithelial phenotype [98]. Here, the Par complex component atypical PKC (aPKC)
phosphorylates S249 of Snail to promote its degradation. This is
dependent on the integrity of the apical Par3-aPKC complex in
polarized cells, revealing apical-basal polarity as a key checkpoint in
restricting EMT in this system [98].
In gastrulating Drosophila embryos, ingressing mesodermal
cells of the ventral furrow exhibit AJs despite co-expressing Snail.
Apical myosin contractility protects AJs from Snail-mediated disassembly, whereas in the absence of myosin contractility, AJs are
disassembled in Snail-expressing cells through post-transcriptional
mechanisms [99]. Thus, mechanical forces may stabilize AJs despite
active Snail expression. Moreover, during development of the Malpighian tubules, the Drosophila renal system, mesenchymal stellate
cells undergo MET to integrate with principle cells to form mature
tubules, which requires basolateral cues from principal cells to allow
stellate cells to integrate and establish apical-basal polarity [100].
Together, these studies suggest that cell and tissue architecture
serve to control EMT and MET. Notably, early studies revealed a
primary role for AJs in establishing epithelial features. For example,
ectopic E-cadherin expression in mouse fibroblasts causes acquisition of Ca
2+
-dependent intercellular contacts [101], and CDH1
overexpression in sarcoma cells causes polygonal epithelial morphology with formation of adherens and gap junctions, although
not of desmosomes or TJs [102]. E-cadherin overexpression in
normally mesenchymal ovarian surface cells causes MET with
re-localization of α-, β- and γ-catenin and F-actin to intercellular
borders, and subsequent tight junction formation and cytokeratin
expression [103]. The mechanism by which AJs function upstream
of establishment of epithelial features may involve interactions
between E-cadherin and the catenins, which are able to function
both at the cell membrane to connect AJs to the actin cytoskeleton,
and also in the nucleus as transcriptional co-factors [104]. For
example, in confluent colon cancer cells in culture, establishment
of AJs sequesters β-catenin at intercellular borders and prevents
Wnt/β-catenin activation, preventing transcriptional activation of
Slug, thus stabilizing E-cadherin transcription [105]. Indeed,
E-cadherin has increasingly recognized roles as a key regulator of
54
John-Poul Ng-Blichfeldt and Katja Ro ¨ per
Numerous studies point to post-translational control of EMT-TF
activity as critical in restricting EMT. GSK3β can phosphorylate
Snail to promote bTrcp-mediated ubiquitination [96]. Conversely,
lysyl oxidase-like 2 (LOXL2) can stabilize the Snail protein via
oxidative deamination of K98/K137 residues [97]. A recent
study identified a new regulation of Snail activity via the apicalbasal polarity machinery of mammary epithelial cells in 3D culture,
which acts to prevent EMT and consolidate the epithelial phenotype [98]. Here, the Par complex component atypical PKC (aPKC)
phosphorylates S249 of Snail to promote its degradation. This is
dependent on the integrity of the apical Par3-aPKC complex in
polarized cells, revealing apical-basal polarity as a key checkpoint in
restricting EMT in this system [98].
In gastrulating Drosophila embryos, ingressing mesodermal
cells of the ventral furrow exhibit AJs despite co-expressing Snail.
Apical myosin contractility protects AJs from Snail-mediated disassembly, whereas in the absence of myosin contractility, AJs are
disassembled in Snail-expressing cells through post-transcriptional
mechanisms [99]. Thus, mechanical forces may stabilize AJs despite
active Snail expression. Moreover, during development of the Malpighian tubules, the Drosophila renal system, mesenchymal stellate
cells undergo MET to integrate with principle cells to form mature
tubules, which requires basolateral cues from principal cells to allow
stellate cells to integrate and establish apical-basal polarity [100].
Together, these studies suggest that cell and tissue architecture
serve to control EMT and MET. Notably, early studies revealed a
primary role for AJs in establishing epithelial features. For example,
ectopic E-cadherin expression in mouse fibroblasts causes acquisition of Ca
2+
-dependent intercellular contacts [101], and CDH1
overexpression in sarcoma cells causes polygonal epithelial morphology with formation of adherens and gap junctions, although
not of desmosomes or TJs [102]. E-cadherin overexpression in
normally mesenchymal ovarian surface cells causes MET with
re-localization of α-, β- and γ-catenin and F-actin to intercellular
borders, and subsequent tight junction formation and cytokeratin
expression [103]. The mechanism by which AJs function upstream
of establishment of epithelial features may involve interactions
between E-cadherin and the catenins, which are able to function
both at the cell membrane to connect AJs to the actin cytoskeleton,
and also in the nucleus as transcriptional co-factors [104]. For
example, in confluent colon cancer cells in culture, establishment
of AJs sequesters β-catenin at intercellular borders and prevents
Wnt/β-catenin activation, preventing transcriptional activation of
Slug, thus stabilizing E-cadherin transcription [105]. Indeed,
E-cadherin has increasingly recognized roles as a key regulator of
54
John-Poul Ng-Blichfeldt and Katja Ro ¨ per